STORM-1042 FIXED Fixed the inventory observers of newly added items.

The problem was caused by an outdated message name stored in LLInventoryObserver::mMessageName and not updated properly in LLInventoryModel::notifyObservers().
The message name used in LLInventoryAddedObserver::changed() was the name of the message most recently passed by LLInventoryModel::notifyObservers(), instead of the name of the latest actually received message. Using the most recent message name in this case fixed the problem.